Conventionally, stainless steel, Ti, Ti alloy, brass, and the like which are soft bases that are easily subjected to partial processing have been widely adopted in decorative articles such as exterior parts for timepieces, necklaces, pendants, and brooches. However, the decorative articles processed with the soft bases have had problems of deteriorations of appearance quality such as scratches due to use thereof, and various surface treatment technologies have been used as measures against the problems.
The above-mentioned exterior parts for timepieces have required high appearance quality and high decorativeness, and as an example of decoration therefor, a black coating film has been formed by a surface treatment technology.
An example of black coating films using surface treatment technologies is a nickel-phosphorus alloy coating film obtained by using a wet plating method (see Patent Literature 1). However, it has been impossible to solve the above-mentioned deteriorations of appearance quality due to scratches because the coating film itself is soft.
Other examples of black coating films include a diamond-like carbon (DLC) film formed by plasma polymerization treatment. The DLC film comprises carbon and has an amorphous structure including hybrid orbitals with an sp3 structure and an sp2 structure. In addition, more generally, 10 to 30 atomic % of hydrogen is contained in the film.
The DLC coating film is harder than a soft base such as stainless steel, Ti, Ti alloy, or brass used as a base for an exterior part for a timepiece or a decorative article and is excellently inhibited from suffering the above-mentioned deteriorations of appearance quality such as scratches due to use thereof (see Patent Literature 2).